Problematic Pompeii Parabola Plan

[Freddie]

How are most of you building your domes ? as high vaults or low vaults? From what I?ve found on this site, a low vault parabola is the desired shape, while the high vault is basically a semi-circle ? the height is the same as the radius.

I?m about to start building the dome for a 39? oven. I want to make my dome a parabolic shape, but not quite as low as the plans call for (about 18? high vs. 15? for a 39? oven). Is there a good way to draw a parabola other than freehand guessing? I?ve found a bunch of web sites showing how to plot a parabola based on a formula, but I can?t find anything showing how to draw a parabola when you know the desired radius and height.

If the low vault parabola proves too hard to figure out or build I?ll go with the high vault using one of the jigs I found on this site. Is the overall performance of a high vault oven significantly different than a low vault oven?

Thanks for the help and info.

Freddie

[dmun]

Re: Problematic Pompeii Parabola Plan

Is the overall performance of a high vault oven significantly different than a low vault oven?

It's said there's not much difference. (I haven't cooked in a low dome) A hemispherical oven has slightly less top-browning, but it's a matter of balance, which you learn when you use your oven. You can always lift your pizza for a few seconds if your bottom is browning faster than your top.

A hemispherical oven has lots of room for fuel, so you can fuel your fire just a few times while heating it up. There's also lots of room for roasts, in retained heat cooking. The low dome is more pizza specific.

The parabola (the conic section) is similar to the catenary (the line formed by a chain suspended in two places). I've used CAD so long that I've forgotten the old methods of generating curves.

[Michael Craig]

Re: Problematic Pompeii Parabola Plan

Freddie,
I too went with a parabola shaped oven. I liked the idea of a flatter dome pushing more heat down on the food. I built my sides completely veritcal, up to about 9''. This allows me to push pots and pans right up to the side with no gap at the bottom and still have volume enough for large roasts.
Now, to make a jig I used a technique I learned from woodworking. You need a piece of tempered hardboard, 1\4'' thick. Cut a strip about 2'' wide and about 5' long. Put a 1\8'' notch in the middle of botton ends. This notch will hold a string at both ends. Attach the string at one end by tying a large knot that will NOT slip through the slot. Make the string a 1' longer than the strip of hardboard. Now, to make a parabola, pull the string through the slot on the other end and bend the hardboard and use the string to keep it in place to make the curve you want. It's going to look like a very crude bow and arrow, without the arrow. Make sure and mark the center of the hardboard, this will mark the peak of your oven. Using this marking tool, you can make your own dome making jig. I used a piece of plywood for mine.
The important thing to remember is to use something that will bend consistently thoughout its length. Good Luck!
I thing a may have pictures in the photo gallery.

[Freddie]

Re: Problematic Pompeii Parabola Plan

Thanks for the info and the ideas. I?m trying to find the perfect balance between a pizza oven and an oven that will cook bread and anything else. My wife is an avid bread maker. She even teaches a class to other women in the neighborhood. I was hoping she would like the idea of a wood fired oven in which she could bake bread. Unfortunately, her opinion is that this is the worst idea I?ve had since trying to make home made rootbeer using real roots and spices (an absolute disaster). I figure if I can find the best shape that will work for both pizzas and breads, then maybe she?ll change her mind (unlikely, but dare to dream).

Freddie

[Wiley]

Re: Problematic Pompeii Parabola Plan

Freddie,
Here's a photo of a couple of pages of a book I own. I have only included the written directions for the first ellipse as the roughness of construction using bricks would make spending the time to draw the more perfect ellipse a "waste of precision" (like measuring a the length of stud for a wall to the 1/1000th of an inch.)

Anyway, here's how to do it with a pair of dividers and straight edge. Just like the old time masons used to do.

Hope it helps,
Wiley

[dmun]

Re: Problematic Pompeii Parabola Plan

I've already been accused of being pedantic this week, but an ellipse (cylindrical section) and a parabola (conical section) are two different things. Ellipses are pretty easy to draw with two nails (focii) a circle of string and a pencil.

[Gromit]

Re: Problematic Pompeii Parabola Plan

I've already been accused of being pedantic this week,

Dmun,

If the sharing of factual information counts as pedantry, then by all means keep it up. Your efforts help keep this a Reality Based Forum.

[Wiley]

Re: Problematic Pompeii Parabola Plan

Right you are dmun, my bad.
Wiley

[Dino_Pizza]

Re: Problematic Pompeii Parabola Plan

Freddie,
I was considering the cylindrical vs conical and the implications of the focii origin , but in the end I went with a "squished ellipse thingie" (Ken's words, not mine) Here is the link to a pdf file I made #132. http://www.fornobravo.com/forum/8/ba...html#post45993

But seriously, you've got a good questions and there are some good answers above. You want your oven as structurally sound as your abilities allow and most of us are only building 1 oven so we want to make pizza and bake bread as well as feed the thing a medium size turkey once a year.

I think an 18-19" tall dome on a 39" oven would be a fine compromise over the perfect sphere. Also, if you make your 1st 3rd of rings more vertical, then arch over (the shoulder rings), then go flat for the last 4 rings at +18 or 19" you would have a tall enough oven for baking & roasting and a flat-ish dome for excellent pizza. There have been quite a few WFO's built in the last few years on this forum and they have all turned out great. If you don't do a perfect half-sphere but do some sort of parabola but without the pointy cone at the apex, (IMHO) you'll have a great dome.

Hope this helps, Dino

[dmun]

Re: Problematic Pompeii Parabola Plan

The ellipse drawing method:

File:Elipse.svg - Wikimedia Commons

[Neil2]

Re: Problematic Pompeii Parabola Plan

"Is there a good way to draw a parabola other than freehand guessing? "

What you want is an ellipse, rather than a parabola. Dmun has a nice sketch on the post before.

As a rule of thumb, the ellipse height should be 85 to 90 % of the radius. Your 39 inch diameter has a radius of 19 1/2 and should have a height of about 17 1/2 inches.

A circle has a single focus point - the centre. An elipse has two focus points, (or in three dimensions, a ring.)

[Neil2]

Re: Problematic Pompeii Parabola Plan

This link has a nice explanation for drawing an ellipse with a specific radius and height:

Double Error Squaring

[Freddie]

Re: Problematic Pompeii Parabola Plan

Thanks to everyone for all the good information and help. Drawing an ellipse seems much easier than coming up with a parabola. I thought the desired shape was a parabola for two reasons: 1) stronger arch, and 2) focuses the heat to the cooking floor. However, if an ellipse works just as well, I?ll go with that design.

What I actually end up with, on the other hand, may be a different story. I?ve found that some things work on paper but in practice they don?t quite work as well. I?ll be using one of the jigs I found on the forum to place the bricks. I think I?ve figured how to make the length adjustable so that I can set each course at a different length and end up with an ellipse or parabola. I?ll shoot for an elliptical shape, but when I start setting the courses I may end up going with the semicircle instead.

Dino ? you hit the nail right on the head. I have no real experience doing anything like this, and I don?t plan on moving any more, so this is my one and only chance to get it right. I?m trying to find the balance between the desired shape and dimension with my very limited skills.

I ordered the Pompeii 110 kit last week but it won?t be delivered for another week or two. This is giving me too much free time to overanalyze things.

[silverman]

Re: Problematic Pompeii Parabola Plan

There is a better wiki on making the parabola, the above doesnt give measurements for the F1 and F2 from the diameter. This one shows a scale, so that F points are well marked.
File:Constructie ellips tuinmanier.gif - Wikimedia Commons

[pacoast]

Re: Problematic Pompeii Parabola Plan

How are most of you building your domes – as high vaults or low vaults?

I'm a big fan of elliptical domes as well, particularly for pizza as they heat a lot more evenly than a hemispherical or parabolic dome. As most vents (doorways) should be about 63% the height of the dome for proper drafting.. that means that there is a trade off between low & high dome ovens. A low dome oven tends to be more efficient & heat evenly. That makes for great pizza & can really cut down on your wood consumption. But it also makes for low doorways and you might not be able to fit a large roast, turkey, spit or grilling rack in your oven.

A high dome leaves you lots of room for larger items like turkeys, but also means that your oven will consume more wood, is a little less ideal for pizza & probably costs a bit more up front for materials as a higher dome probably means a bigger oven overall.

There is also an issue of how much thermal capacity you want in your oven. If pizza is the main use, a thinner wall will let your oven heat up a lot quicker. If bread is more important, then thick walls hold a steady temperature better and allow the option of many batches from a single firing. Which is better depends on how you see yourself using your oven. For myself it's 90% pizza/10% baguettes, sandwiches & bread, so I'd be looking for a very low, elliptical dome.

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